Endoscopes, that is, instruments for viewing the interior of volumes not accessible to direct examination, are shown by prior art extending back many years. A number of different classes of endoscopes are shown in the prior art.
In a first class of endoscopes, an image formed by an objective at the distal tip of a rigid tubular member is transferred by a transfer module comprising a sequence of refractive optical elements to an image-forming device at the proximal end of the tube. See, for example, Broome U.S. Pat. No. 5,341,240. The image-forming device may include an optical eyepiece for direct viewing, or an electronic imaging device and associated circuitry for providing a visible image on an electronic display screen.
The prior art also teaches endoscopes wherein the image formed by an objective at the distal tip of the endoscope is transferred by a fiber optic bundle to an image-forming device at a proximal end of the endoscope. Employment of the fiber optic bundle in lieu of the refractive elements in a transfer module disposed in a rigid tube allows some flexibility of the shaft of the endoscope, which is of particular value in medical applications. However, the ultimate image quality is severely limited by the use of the fiber optic bundle to transfer the image from the objective to the image-forming device.
A third class of endoscopes is shown in, for example, U.S. Pat. No. 5,305,736 to Ito, wherein a solid state imaging element, typically a charge-coupled device (CCD) imaging element, is juxtaposed to the objective, so that an image signal is formed at the distal tip of the endoscope. As only wires are needed to transfer the image signal from the tip of the endoscope to means for display, the endoscope can be relatively flexible.
Ito also teaches the transmission of illuminating light from a source located at the proximal end of the endoscope to its distal tip by a fiber optic bundle. A dispersing optical element is provided at the tip of the fiber optical bundle, causing the light emitted by the fiber optic bundle to diverge, illuminating a relatively broad field of view.
In its preferred embodiment, the present invention relates generally to an endoscope of the class described in the Ito patent, that is, the present invention also relates to an endoscope wherein an image signal is formed by a solid state imaging element juxtaposed to the objective. Also as in Ito, an optical conduit, such as a fiber optic bundle, carries light from a source external to the proximal end of the endoscope to its distal tip, and a dispersive optical element is used to illuminate a wide field of view. However, the present invention makes numerous improvements over the endoscope shown in Ito, and certain aspects of the invention described herein have applicability beyond this preferred embodiment.
Other prior art references generally relevant to the subject matter of the present invention include Nishioka U.S. Pat. No. 4,824,225, teaching a particular illumination optical system for an endoscope. Nishioka teaches use of an aspheric lens for dispersing light from the distal tip of a fiber optic bundle, so as to illuminate a relatively wide field of view.
A number of additional patents also address the question of providing sufficient illumination to the field of view of an endoscope. See, for example, Matsumoto U.S. Pat. No. 5,491,765 showing a light source for an endoscope. The light source includes a parabolic reflector directing light from a lamp onto an aspheric element for focusing the light onto the input end of a fiber optic bundle, which then transmits the light to the distal tip of the endoscope for illuminating a field of view.
Wood U.S. Pat. No. 5,331,950 teaches a light source for an endoscope wherein a relatively low-powered metal halide arc discharge lamp is disposed at the focus of a reflector for concentrating light on the proximal end of a fiber optic bundle. This solution is technically useful in that such metal halide discharge lamps provide substantial illumination; however, such lamps require costly power supplies and are themselves relatively expensive.
It has been known for many years that, at least in theory, diffractive optical elements can be advantageously substituted for many types of refractive optical elements, for example, to focus light from a field of view in order to form an image. In this connection, a refractive optical element is one in which rays of light are bent at the interfaces between differing transmissive media. The degree of bending is proportional to the relative velocities of light in the two media, and is expressed by Snell's Law. Lenses in common use are typically refractive; for example, Ogasawara et al U.S. Pat. No. 5,619,380 discloses an objective for an endoscope employing refractive lens elements.
So-called "Fresnel" lenses are also refractive. In Fresnel lenses, the smooth surfaces of conventional refractive lenses are divided by grooves into a number of smaller areas, typically concentric circular zones, and the two optical surfaces of the lens are effectively collapsed toward one another. The advantage is that the overall thickness, weight, and cost of the lens are reduced. The width of the zones, i.e., the spacing of the grooves, is many hundreds of wavelengths of the light to be focused. Claytor U.S. Pat. No. 4,787,722 shows such Fresnel lenses.
By comparison, diffractive optical elements typically comprise a number of parallel lines or grooves spaced on the order of the wavelength of the light to be focused. Light rays are bent through an angle which varies as a function of the wavelength of the light and the spacing of the grooves. If the lines are spaced uniformly, the structure is commonly referred to as a diffraction grating, and the light is simply bent differentially as a function of wavelength; that is, white light is broken into its constituent colors. This phenomenon accounts for the spectral patterns seen when one inspects a structure including closely spaced grooves, such as the surface of a conventional "CD-ROM" optical storage device.
If the spacing of the grooves is varied, the diffractive effect may be used to form an image by bending light incident from various directions differentially, i.e., so that light from different parts of a field of view is directed onto corresponding portions of an image plane. Moreover, if the depth of the grooves is also varied, the diffraction efficiency (the amount of the incident light that is diffracted into a particular "order") can be optimized, to form a substantially uniformly illuminated image.
The basic theoretical underpinnings of diffractive optics are shown in numerous references. See, for example, Sommerfeld, Optics, Section D, "Phase Gratings", pages 228-233 (1964); Miyamoto, "The Phase Fresnel Lens", JOSA, Vol. 51, No. 1, pages 17-20 (1959); Swanson et al, "Infrared Applications of Diffractive Optical Elements", SPIE Holographic Optics: Design and Applications, Vol. 883, 155-162 (1988); Faklis et al, "Optical Design of Diffractive Lenses", Photonics Spectra, 205-208 (November 1991); Faklis et al, "Diffractive Lenses in Broadband Optical System Design", Photonics Spectra, 131-134 (December 1991).
Various U.S. patents teach diffractive lenses and other diffractive optical elements, both in general and for specific purposes. Tomlinson et al U.S. Pat. No. 3,814,498 teaches that by selecting the spacing and depth of grooves, an optical grating can be caused to focus light. Hettrick U.S. Pat. No. 4,798,446 is generally similar, as is Aoyama et al U.S. Pat. No. 5,132,843. Futhey U.S. Pat. No. 4,936,666 teaches specific improvements in manufacture of diffractive lenses. Three Chen patents, U.S. Pat. Nos. 5,151,823, 5,044,706, and 5,257,133, show various optical systems employing diffractive elements, none of these relating directly to endoscopes. Maruyama et al U.S. Pat. No. 5,629,799 discusses diffractive elements at some length, and provides specific designs for objectives using at least one diffractive element, in some cases provided over an underlying aspheric surface, for laser diode reading systems for optical disk drives. Finally, Baker U.S. Pat. No. 5,261,904 teaches a laser catheter for ablating atherosclerotic deposits in veins and the like, having a diffractive grating for beam shaping.
However, while as noted there exists ample technical literature relating to diffractive optical elements in general, and while various patents address specific systems employing such elements, no prior art presently known to the inventor discusses use of such diffractive elements in endoscopes per se.
Manufacture of a suitable endoscope involves consideration of numerous optical and related constraints. It will be apparent, of course, that substantial illumination must be provided by a lamp and dispersed into the field of view at the distal tip of the endoscope in order that the objective can collect sufficient light to form a useful image. In general, the more light delivered, the better the quality of the image. However, as noted, the brightest light sources available for such use, that is, short-arc lamps such as xenon lamps, or the metal halide lamps used in the Wood patent, are too expensive for certain uses. It would therefore be desirable to provide an optical system for an endoscope sufficiently efficient (including both the optics for transferring light from the source to the tip of the endoscope, and the image-forming objective) that a less expensive lamp could be used.
Reduction in cost of endoscopes is always desired; of course, in the medical context, reduction of cost means that an instrument will be affordable for use in more types of diagnostic and therapeutic procedures, which can ultimately lead to an improvement in patient care. Hence, it is significant to reduce the cost of the components employed in the endoscope.
Similarly, the smaller the endoscope the better, particularly for medical use, so as to be received into various cavities or incisions allowing the patient's anatomy to be examined. Given that larger optical elements are in general able to collect more light, to combine a relatively low cost illumination source with an efficient objective capable of collecting sufficient light to form high quality images while remaining small in diameter poses a significant technical challenge not met by any prior art endoscopes known to the inventor.